Dunbar's number is a value significant in sociology and anthropology. Proposed by British anthropologist Robin Dunbar, it measures the "cognitive limit to the number of individuals with whom any one person can maintain stable relationships". Dunbar theorizes that "this limit is a direct function of relative neocortex size, and that this in turn limits group size ... the limit imposed by neocortical processing capacity is simply on the number of individuals with whom a stable inter-personal relationship can be maintained."

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Research background

Primatologists have noted that, due to their highly social nature, non-human primates have to maintain personal contact with the other members of their social group, usually through grooming. Such social groups function as protective cliques within the physical groups in which the primates live. The number of social group members a primate can track appears to be limited by the volume of the neocortex region of their brain. This suggests that there is a species-specific index of the social group size, computable from the species' mean neocortex volume.

In a 1992 article, Dunbar used the correlation observed for non-human primates to predict a social group size for humans. Using a regression equation on data for 38 primate genera, Dunbar predicted a human "mean group size" of 147.8 (casually represented as 150), a result he considered exploratory due to the large error measure (a 95% confidence interval of 100 to 230).

Dunbar then compared this prediction with observable group sizes for humans. Beginning with the assumption that the current mean size of the human neocortex had developed about 250,000 years BCE, i.e. during the Pleistocene, Dunbar searched the anthropological and ethnographical literature for census-like group size information for various hunter-gatherer societies, the closest existing approximations to how anthropology reconstructs the Pleistocene societies. Dunbar noted that the groups fell into three categories — small, medium and large, equivalent to bands, cultural lineage groups and tribes — with respective size ranges of 30-50, 100-200 and 500-2500 members each.

Dunbar's surveys of village and tribe sizes also appeared to approximate this predicted value, including 150 as the estimated size of a neolithic farming village; 150 as the splitting point of Hutterite settlements; 200 as the upper bound on the number of academics in a discipline's sub-specialization; 150 as the basic unit size of professional armies in Roman antiquity and in modern times since the 16th century; and notions of appropriate company size.

Dunbar has theorized that 150 would be the mean group size only for communities with a very high incentive to remain together. For a group of this size to remain cohesive, Dunbar speculated that as much as 42% of the group's time would have to be devoted to social grooming. Correspondingly, only groups under intense survival pressure, such as subsistence villages, nomadic tribes, and historical military groupings have, on average, achieved the 150-member mark. Moreover, Dunbar noted that such groups are almost always physically close: "... we might expect the upper limit on group size to depend on the degree of social dispersal. In dispersed societies, individuals will meet less often and will thus be less familiar with each, so group sizes should be smaller in consequence." Thus, the 150-member group would only occur because of absolute necessity, i.e. due to intense environmental and economic pressures.

Dunbar proposes furthermore that language may have arisen as a "cheap" means of social grooming, allowing early humans to efficiently maintain social cohesion. Without language, Dunbar speculates, humans would have to expend nearly half their time on social grooming, which would have made productive, cooperative effort nearly impossible. Language may have allowed societies to remain cohesive, while reducing the need for physical and social intimacy.

Dunbar's number has since become a major meme of interest in anthropology, sociology, statistics, and business management. As with many theoretical values, it has occasionally been abused and mistaken as a "magic number".

It has also been popularized as the monkeysphere, a neologism coined by David Wong in an article which introduces this concept in a humorous manner.

In its popularization, the research of Dunbar and others is taken as an upper bound of the number of fellow humans that an individual can view as being "truly human". In this form, the "monkeysphere" functions as a reductionistic and biologistic explanation for why humans can treat some humans with consideration and other humans indifferently or even inhumanely.

Some example explanations using the notion of a monkeysphere are:

"Whenever you make new close personal friends, you have to drop some old personal friends to make room for them in your monkeysphere."

"The reason that the people in village X don't mind doing Y to the people in village Z is because the people in village Z are not in the monkeysphere of people in village X."

"Because the number of people in that department exceeded 150, which is the size of the human monkeysphere, they had to split the department into two."